help! My truck won't lock. Go figure

[Peregrine]

Ok, I've never seen this happen and it's really got me puzzled. The guy I got my truck from didn't have a door lock key so I never tried to lock it. I call a locksmith, have a a key made and the lock re-coded just in case he did have a key and want's to steal it back. So, door locks, unlocks, all keys work. Well, passenger side lock is different, but I digress. So I lock my truck and go inside for an hour, my dad gets home and I look out the back door and he's standing there with my passenger door open. I'm like *jaw drop* "How'd you open that? It's lock!" Well, apparently, when you press the button on the outside it unlocks the freakin door just like if you open it from the inside when it's locked. Can you change something so it does that? If someone didn't have a key it makes sense to do, but sure doesn't help me keep my truck locked.